Wet-suit ??

You are confused about the rules. The rules you are looking at are for an event when water temperatures are warm enough for the race to be declared "not wetsuit legal". It varies a bit with different countries and sponsoring organizations, but basically you would be looking at water temps below about 75-77 degrees for amateurs. (Forgive me--I don't know the exact temps off the top of my head as I mostly race in SE Asia where you almost never wear a wetsuit, occasionally in North America or elsewhere, but water has always below 72, not borderline, there). You can look up the borderline temperature on the organizer's website.

When a race IS "wetsuit legal", you CAN wear a neoprene wetsuit, and most people would as it gives you a speed boost due to the additional buoyancy, as well as keeping you warm. If the race has really cold water, it may actually require people to wear wetsuits for safety reasons. Otherwise they would be optional until the temps hit the organizer's "borderline" temperature. Above that, you have to swim in your trisuit or swimsuit, no wetsuits allowed. When temps are right below the borderline, most triathletes will still show up in full wetsuits, but you may find some stronger swimmers electing not to wear them for reasons like comfort or (in a short race like a sprint) wanting to save time in transition.

For the "in it to win it" crowd who want to spare no expense, there are suits called swim skins made of nylon or lycra that go on over the trisuit and can be worn when a race is not wetsuit legal. They don't aid in buoyancy, but can improve your hydrodynamics and help you pick up a few seconds, maybe. If thinking of the latter, check the materials. I nearly bought a swim skin on sale in Australia that was NOT legal for non-wetsuit races under current rules. Apparently these have changed since the product was developed, but people still use them for training. The merchant was nice enough to let me know that before I snapped it up as a bargain!